Atk is a parameter that describes the ATK of a card.
Acceptable forms:
Atk:300 ---exactly 300 ATK
Atk:>300 ---greater than 300 ATK
Atk:>=300 --greater than or equal to 300 ATK
Atk:<300 ---less than 300 ATK
Atk:<=300 ---less than or equal to 300 ATK
Atk!:300 ----NOT 300 ATK
Spell and Trap cards (except Trap Monsters, etc.) will always test False for any of the above forms.
Also see OriginalAtk